This application is related to Japanese application No. Hei 11(1999)-021040 filed on Jan. 29, 1999, whose priority is claimed nnder 35 xc2xa7USC, the disclosure of which is incorporated by reference in its entirety.
1. Field of the Invention
The present invention relates to a device and a method for reproducing a magnetic signal for magnetically performing reproduction while heating a recording medium. In particular, it relates to a device and a method for reproducing a magnetic signal capable of suppressing a crosstalk sent from an adjacent track and the like.
2. Description of the Related Art
In recent years, multimedia technologies have been developed and a memory device having a larger capacity has been highly demanded to deal with information having a large capacity. In particular, techniques for densifying a rewritable optical disc, a rewritable magnetic disc and a rewritable magnetic tape have been actively investigated.
Among them, there has been proposed a method for reproducing a magnetic signal capable of performing high density recording and reproduction (hereinafter referred to as a thermally assisted magnetic signal recording and reproducing method). In this method, a magnetic recording medium which changes its magnetic characteristics depending on a temperature is irradiated with light to locally provide a heated region on the medium and selectively and magnetically record or reproduce information only in the heated region.
An example of the thermally assisted magnetic signal recording and reproducing method described in Japanese Unexamined Patent Publication No. Hei 4(1992)-176034 will be described below.
In this example, a ferrimagnetic substance in which a magnetization becomes zero at a temperature close to a room temperature (hereinafter referred to as a magnetic compensation point) is used as a recording medium. For recording, a light beam is irradiated on a region in the recording medium where the recording is to be performed so as to heat up to around the Curie temperature, and an external magnetic field is applied by means of a recording head. Thus, information is recorded. On the other hand, for reproduction, a light beam is irradiated to heat on a region of the recording medium where the reproduction is to be performed. Thereby, the magnetization of the region is enhanced to detect a magnetic flux leaking therefrom with a reproducing head.
However, in order to implement the high density reproduction by the thermally assisted magnetic signal reproducing method of the above-described prior art (Japanese Unexamined Patent Publication No. Hei 4(1992)-176034), a magnetization in a region adjacent to a region where the reproduction is to be performed (hereinafter referred to as a reproducing portion) must be reduced as much as possible. For this reason, it is necessary to control a medium temperature in the adjacent region to close to the magnetic compensation point as accurately as possible. If the medium temperature is shifted from the magnetic compensation point, information transmitted from the adjacent region is superposed on a reproducing signal. Consequently, an accurate reproducing signal cannot be obtained. A specific example will be described below.
FIG. 12 is a schematic view showing an information recording state on a magnetic recording medium (a bit pattern). Illustrated is magnetically recorded information which is recorded on three tracks by a magnetization into three kinds of different patterns. There is taken the case where only the magnetically recorded information on a central track is reproduced by using a reproducing head (for example, an MR head or the like) having a signal detecting region 100 for three track widths. It is assumed that the magnetic recording medium is a common n-type ferrimagnetic substance having the temperature characteristics for exhibiting a magnetization shown in FIG. 11 and that the magnetic compensation point thereof is almost coincident with a temperature of the medium in the vicinity of a portion provided immediately under the reproducing head in a state where a temperature is not raised.
For reproducing only the central track shown in FIG. 12, a temperature of only the central track is raised with a light beam or the like. If a temperature of a heated region 101 (central track) is set close to T3 (see FIG. 11) and temperatures of regions which are not heated (both adjacent tracks) are set close to a magnetic compensation point T1 (see FIG. 11), the magnetization of the magnetic recording medium is sufficiently enhanced in the heated region 101 (central track) and the magnetization of the non-heated regions (both adjacent tracks) is fully reduced. Consequently, a reproducing signal includes only a magnetic signal in the heated region as shown in FIG. 13. Therefore, only the magnetically recorded information in the central track can selectively be reproduced.
However, when an ambient temperature increases, temperatures in the both adjacent tracks, that is, the non-heated regions are changed and shifted from the magnetic compensation point, thereby causing the magnetization of the regions. In this case, information recorded on these regions is mixed in the reproducing signal. For example, when the temperatures in the both adjacent tracks are raised from T1 to T2, respectively (see FIG. 11) and the magnetization of the reproducing signal becomes such an extent that contributes to the heated region by 1/2, a reproducing signal corresponding to a bit pattern in FIG. 12 is obtained as shown in FIG. 14. More specifically, a reproducing signal in which signals sent from the both adjacent tracks are superposed as noises is resulted. In such a case, magnetically recorded information on the central track cannot be accurately reproduced.
As described above, in order to perform high density reproduction in a region which is much smaller than a magnetization detecting region of the reproducing head according to the thermally assisted magnetic signal reproducing method which has been conventionally proposed, it is required that a non-heated region should have a sufficiently small magnetization. In other words, a shift from the magnetic compensation point of the medium temperature should be small. However, even in a working environment which is supposed as very general one, a change in an ambient temperature ranging several tens xc2x0 C. is taken place depending on seasons, time or a place for use. Further, a large number of heating sources, for example, a reproducing head, a motor, an electronic circuit and the like are present in the vicinity of the medium. For these reasons, it is not easy to keep the temperature in the non-heated region of the medium close to the magnetic compensation point.
Moreover, there have also been technical problems in that the magnetic compensation point of the ferrimagnetic substance is sensitive to a composition ratio thereof, and it is hard to fabricate a medium having a constant magnetic compensation point over the whole recording regions with high productivity.
As described above, to perform high density reproduction in the above-mentioned thermally assisted magnetic signal reproducing method according to the prior art, it is necessary to fabricate a magnetic recording medium having an almost constant magnetic compensation point over the entire recording regions by strictly controlling a composition ratio thereof. Further, it is also necessary to monitor and control a temperature for a very minute region, i.e., a magnetization detecting region on the reproducing head in order to suppress the influence of tracks adjacent to a track to be reproduced, that is, a crosstalk. This has been very difficult to carry out technologically.
The present invention provides a magnetic signal reproducing device comprising heating means for locally heating a reproducing region of a magnetic recording medium, first reproducing means for reading magnetic information in the reproducing region in a first temperature state to obtain a first reproduced signal, second reproducing means for reading magnetic information in almost the same region as the reproducing region in a second temperature state which is different from the first temperature state to obtain a second reproduced signal, and correcting means for correcting the first reproduced signal based on the second reproduced signal to obtain an information reproducing signal representing recorded information in the reproducing region.
According to the present invention, a medium temperature is changed to obtain two reproducing signals in almost the same region on the magnetic recording medium and a correction processing is carried out based on these signals to obtain a reproducing signal. Therefore, it is possible to obtain a reproducing signal free from being affected by a magnetic signal which is possibly mixed from the adjacent tracks.